Enigmail is an extension for the mail client of Mozilla, Netscape 7.x, and Mozilla Thunderbird that allows users to access the authentication and encryption features provided by the popular GnuPG software. It can encrypt/sign mail when sending, decrypt/authenticate received mail, and import/export public keys. It supports both the inline PGP format and the PGP/MIME format, which can be used to encrypt attachments, and is cross-platform, although binaries are supplied only for a limited number of platforms. Enigmail uses inter-process communication to execute GPG to carry out encryption/authentication.
Bork is a very small, cross-platform file encryption utility using a password-based RC4 algorithm. It is designed to be included in source code form along with the files it encrypts for long-term storage (eg on CD-R). Its minimal external dependencies are designed to make it fairly futureproof.
Java Security Component Framework (JSCF) is a pluggable security component system with a JDBC-like interface. It allows connections to be made to registered security components with defined operations used mainly for mail, such as encrypting, decrypting, verifying, and signing streams. An implementation that works with the popular gpg program is included.